Date   : Mon, 19 Jul 2004 18:21:26 +0200
From   : John Kortink <kortink@...>
Subject: GoMMC and sideways RAM systems

Wouter Scholten brought this to my attention, and considering
it may be an important issue to those wanting to use GoMMC,
the following :

The GoMMC code writes to the sideways ROM space it's in. Much
like anything could, were it in a 'standard' RAM bank (as in :
selected exclusively by ROMSEL).

This presents no problems if ROMSEL is always used to point
to the read-enabled as well as write-enabled sideways RAM/ROM
bank. Simple SWRam systems and the Master SWR system use this.

However, on more 'fancy' sideways RAM systems, the write-enabled
bank can usually differ from the read-enabled (ROMSEL selected)
bank. This means that GoMMC, when active, can accidentally corrupt
the contents of the then write-enabled bank. Also, the 'select bank,
*LOAD rom_image 8000' shortcut often provided by 'fancy' systems is
rendered useless, since GoMMC will be active during the *LOAD (at
least if it is serving the active filing system), and will have to
fault access to its write-sensitive areas.

GoMMC can be used quite safely in the presence of such sideways
RAM systems. But the user must live with two caveats :

- Never leave a sideways RAM bank with sensitive contents
  write-enabled (i.e. after loading a ROM into it).
- *LOAD directly to a RAM bank is not possible.

Those who can't live with these two caveats, please steer clear
from GoMMC. No fix can and/or will be provided.


John Kortink

-- 

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